1. Field of Invention
This invention relates to devices for reducing noise and absorbing vibrations generated by an electric motor built into the propulsion pod of a ship.
2. Description of Related Art
Surface shipping is propelled in various ways, including in particular: mechanical or electrical propulsion, known as classical propulsion, comprising one or more lines of shafts inside the hull with one or more propellers at the ends and one or more rudders physically independent of the propulsion system; and pod propulsion wherein one or more electric motors associated with one or more lines of shafts are built into a fixed or swiveling pod outside the hull of the ship, with the pod thus acting as an active rudder since it is able to rotate relative to the hull. The first industrial developments of this electric pod propulsion method are very recent, dating back only about ten years. This system, also known to specialists by the English term xe2x80x9cpodxe2x80x9d, is a significant breakthrough for propulsion of passenger liners, ice-breakers, and other ships. It is of great interest in the navy and merchant navy as it relates to electric propulsion applications and to applications replacing certain mechanical applications.
This electric propulsion system for ships is highly maneuverable and has good dynamic performance. Its motor, placed inside a submerged pod that pivots 360xc2x0, is coupled directly to a very short propeller shaft. Such a propulsion system can develop powers as high as 25 MW. In this technical area, several reductions to practice have already been proposed. Pod type propulsion systems for ships comprised in particular of a swiveling pod containing an electric motor driving one or more propellers rotationally through a shaft are known in the art.
For currently known pods, the electric motor is generally mounted rigidly in the pod, particularly by shrink fitting or crimping. This type of mount allows some of the heat given off by the electric motor to be evacuated by conduction between the body of the pod and the surrounding seawater. The motor is also cooled by a cooling circuit and/or by ventilation as described for example in international patent applications PCT WO 97/49605, WO 99/05023, WO 99/05024, and WO 99/36312.
Such a mount for the electric motor in the pod has a major drawback linked to propagation of vibration and noise emitted by the electric motor. This mount is favorable to cool the motor by creating a heat bridge between the motor, which heats up, and the water surrounding the pod. However, at the same time it creates a sound bridge between the motor, which is also a significant noise source, and the outer envelope of the pod which radiates into the surrounding water and transmits vibrations to the ship structure. Hence it is highly unfavorable from the acoustic detectability standpoint. because these vibrations are transmitted directly to the pod without attenuation and can thus propagate in seawater, or toward the hull of the ship through the arm linking the pod to the ship. In fact, the electric motors of all the pods currently on the market are rigidly mounted in the pod and often crimped, precisely to favor heat exchange with the surrounding water, thus reducing the need for other cooling means.
Several designs for large second-rank naval vessels are currently planned. The principal navies of the world are also looking at the possibility of providing some frigates with this electric pod propulsion method. The particular and highly useful application of pod propulsion on naval vessels imposes acoustic detectability requirements that cannot be met by existing pods, due in particular to the type of electric motor mount in the pod normally found in these propulsion systems.
The advertising of pod manufacturers boasts of the silent nature of this type of propulsion. This argument is also taken up by some of their customers, such as shipyards that build passenger liners. This point requires clarification. One of the intrinsic features of pods is that the propeller can be located forward of the pod and thus act as a driving propeller in a relatively undisturbed hydrodynamic flow, as there is no impediment to flow in front of it. On the other hand, a classical push propeller attached behind a line of shafts exiting rearward from a hull encounters a highly perturbed flow. Because of this favorable arrangement of the propeller, the pressure fluctuations on each propeller blade when the propeller rotates are minimal and consequently transmit only very small pressure forces to the parts of the ship""s bottom near the propeller. Since these pressure impacts generate noise on the structure on the ship, the pod appears by nature to be favorable to a reduction in noise of hydrodynamic origin in the ship.
Patent EP 1010614 describes a pod for a surface vessel having in particular an electric motor cooled by a liquid carried in a circuit connected to a heat exchanger located in one end of the pod. The stator of the motor is mounted on an elastic element to damp vibrations generated by the motor. The heat exchanger plus the cooling circuit placed around the motor take up a great deal of space inside the pod so that there is no room for air circulation around the motor. Moreover, no details are given as to the arrangement of the elastic element, which is connected only to the stator of the motor.
U.S. Pat. No. 6,116,179 teaches the mounting of machinery inside a ship using levitation means comprised of electromagnetic devices located between the machinery and the hull of the ship. By varying the electromagnetic forces, contact-less centering and holding of the machinery in the hull are achieved. These forces are also controlled to reduce the noise radiated by the hull. This arrangement requires electromagnetic isolation of these devices from the perturbations emitted by the machinery, thus forming a shielded enclosure that does not allow sufficient radial air circulation between the machinery and the hull. Moreover, maintenance of the machinery with the contact-less electromagnetic devices is a complex technique that is difficult to master.
Patent EP 0533359 describes an electric motor for driving a ship. This is a disc motor with permanent magnets and a large number of small converters located on the front and rear faces of the motor, an elastic coupling attached to the rotor, and means for controlling the converters associated with a particular internal motor design to reduce noise. The motor is mounted on sound insulators by lugs attached to the outer surface of the motor frame. The motor is cooled by fluid circulating in the stator. Converters located on the front and rear faces of the motor are cooled by cold water circulation plates. Because of manufacturing constraints for disc motors, passage cross-sections large enough for an air flow ensuring good motor cooling cannot be provided.
The goal of the present invention is to overcome the above-described drawbacks by providing a device capable of absorbing a sufficient portion of the vibrations generated by operation of the electric motor to meet the acoustic detectability requirements (noise radiated into the water) of the vessel and its propulsion system imposed on naval vessels. Another goal of the invention is to ensure continuous, highly stable physical maintenance of the motor in the pod without creating an impediment to circulation of the cooling air used to cool the motor. The present invention also has the goal of improving acoustic comfort on board the ship. Another goal is to reduce transmission of airborne noise from the motor to the pod without increasing the pod diameter, which would adversely affect the efficiency and noise of the propeller. Hence, the goal of the invention is to reduce the noise level emitted by the electric motors of the pods, whether toward the vessel itself or into the environment.
For this purpose, the invention relates to a device for absorbing vibrations generated by an electric motor built into a propulsion pod of a ship. The motor is cooled by the circulation of radial air flow in the pod and drives rotationally at least one propeller through a transmission shaft. The device is disposed in the pod to position and hold the motor inside the pod, filter the vibrations emitted by the motor, and channel the motor-cooling air flow circulation.
In one exemplary embodiment, the means are comprised of decoupling studs located between the motor and the interior wall of the pod. In another exemplary embodiment, each decoupling stud is attached to a first element attaching to the motor and to a second element attaching to the inside wall of the pod. The decoupling studs may be made of elements that are active and/or passive relative to the vibrations of motor. The decoupling studs may also be oriented in two essentially perpendicular directions or may be radially symmetrical relative to the axis of the motor. The first element attaching the motor to the decoupling stud is comprised of a mounting lug or a brace or directly by the motor flange. The second element attaching the decoupling stud to the interior wall of the pod is comprised of a mounting lug or a brace. Preferably, the electric motor is connected to the transmission shaft through an elastic coupling.
This device has the advantage of providing a particularly quiet electric propulsion pod whose outer shape and propulsion capabilities are unchanged relative to existing systems. Another advantage resides in the quality of the motor stability inside the pod, associated with optimized cooling air circulation over all the various faces of the motor.
It is important for certain vessels to reduce the noise level or vibration level on board for the comfort of the passengers and crew, and to facilitate on-board work. It is just as important to reduce the noise level transmitted to the environment. This applies to airborne noise transmission in order not to inconvenience populations in the vicinity of ports and canals. It also applies to transmission into the marine environment. The noise from a ship can propagate over very long distances at sea. It is a major source of indiscretion for a military vessel, can seriously interfere with measurements by scientific, oceanographic, or geophysical research vessels, and may have a non-negligible effect on numerous marine animals by interfering with the acoustic signals they emit or pick up for direction-finding or recognition of other animals.
Other features and advantages of the invention are described in or are apparent from the following detailed description of the preferred embodiments.